The invention relates to vessels for biomass and particularly to vessels having internal structures to prevent excessive compression of the biomass within the vessel.
Reactor vessels are used to treat biomass to dissolve carbohydrates or lignin or other components of the biomass to produce pulp, fuels or chemicals. Reactor vessels may be large, vertically oriented and pressurized. A typical reactor vessel may have a height of greater than 100 feet (33 meters) and a diameter of at least 30 feet (10 meters). A reactor vessel may be cylindrical with a closed top and bottom sections. An inlet to reactor vessel may be at the top section and may include a top separator device to remove a portion of liquor from the biomass as it enters the reactor vessel. An outlet from the reactor vessel may be in the bottom section. The biomass moves vertically down though the reactor vessel from the inlet to the outlet. The retention period of the biomass in the reactor vessel is dependent on the treatment performed in the vessel and may be several hours, such as two to six hours. The pressure in the vessel may be increased substantially above atmospheric pressure such as by adding steam to the vessel, such at the top section of the vessel. Hot water or steam may be injected into the vessel to add heat energy to the biomass in the vessel and achieve a desired temperature of the biomass in the vessel.
Biomass from annual plants tends to have low bulk density and large specific surface area, as compared to wood chips. Due to the low initial bulk volume and large specific surface, annual plant biomass tends to be more compressible than wood chips. At the bottom of a reactor vessel, biomass especially when saturated with a liquid, can become highly compacted as compared to the compaction of wood chips in a reactor vessel for pulping. The high level of compaction of biomass tends to increase the risk that regions of the biomass will stagnate in the reactor vessel and other regions will form columns of fast moving biomass in the vessel.
The high compaction of the annual plant biomass can apply substantial mechanical loads in the lower portion of the reactor vessel and, particularly, on a discharge device at the bottom of the vessel. These high mechanical loads may increase the energy required to operate the discharge device, such as by increasing the power needed to rotate a scraper in the bottom of the reactor. If excessive, the high compaction may inhibit the operation of the discharge device. Further, the high compaction may damage the discharge device, such as by bending the arms of a scraper.
The high compaction may also prevent the flow of biomass through the reactor vessel. The high compaction may so compress the biomass into a solid mass that it does not flow through the reactor. Further, the compaction may create compressed regions of biomass in the vessel that do not flow downward through the vessel.
Large reactor vessels processing wood chips are common and well know to produce pulp for paper making and other wood-based products. The liquor content in a reactor vessel processing wood chips are relatively high. The high liquor content aids in moving the wood chips down through the vessel at a uniform rate, and helps avoid regions of stagnate chip flow and fast moving columns of chips. However, high liquor content has drawn backs, such as reducing the amount of chips that move through the vessel and increasing the volume of liquor and chips to be pressurized and heated.
Biomass flows through reactor vessels with substantially less liquid/liquor that is conventionally used to process wood chips in pulping. Maintaining a low water content in the reactor vessel is generally desired to maximize the concentration of the released sugars and other desired components from biomass, especially annual plant biomass. Maintaining a low liquid content, e.g., water content, also reduces the amount of energy needed to elevate the temperature in the reactor vessel and suppresses steam generation in the vessel.
Biomass from annual plants absorbs substantially more liquid per dry weight unit than do wood chips. The high adsorption of liquid in the biomass reduces the amount of free liquid available to lubricate the flow of biomass through the reactor vessel. Annual plant biomass becomes saturated as it absorbs the water or other liquid added to a reactor vessel. When saturated with a liquid, annual plant biomass has about the same wet density as a liquid saturated wood. The weight of saturated biomass creates large downward forces due to gravity in a reactor vessel.
The amount of free liquid in a biomass filled reactor vessel tends to be low because of the low ratio of water to biomass and the high absorbance of liquid by the annual plant biomass. As a result of the low amount of free liquid, the liquid level, to the extent it exists, in a biomass filled reactor vessel is at a relatively low elevation in the vessel and well below the level of the biomass. The amount of compaction of biomass at the lower elevations in the reactor vessel tends to be high due to the large height difference between the level of biomass and the liquid level. The biomass does not float in the reactor vessel because of the low liquid level. The lack of floating further compacts the biomass at the bottom of the reactor vessel.
Reactor vessels generally have a discharge at their bottom. The discharged device may be a scraper, screw conveyor or other device which promotes the continuous removal of biomass from the reactor. The biomass at the bottom of the reactor vessel may be in a liquid phase if there is free liquid in the vessel. If there is substantially no free liquid in the vessel, the biomass is in a solid phase at the bottom of the vessel. The discharge device in the reactor vessel may need to be suitable to discharge biomass in either a liquid or solid phase. The discharge device may also need to be capable of operating with the compacted biomass at the bottom of the reactor vessel.
Prior attempts to release excessive compression forces in a large pressurized reactor vessel include adding flow rings in a wood chip vertical reactor vessel, such as shown in U.S. Published Patent Application 20030201080. In a reactor vessel processing annual plant biomass, the compaction can be excessive such that the material can hang up on the conical flow rings that extend around a vessel. When the biomass exhibits high unconfined yield strength and arching dimensions, the biomass can hang up on the conical flow ring inserts in a reactor vessel. The result will be channels of biomass flows in the vessel, stagnant pockets, arches of biomass in the vessel, and intermittent or permanent stoppage of the flow of biomass through the reactor vessel.
Reactor vessels having sidewalls that converge in one dimension have been used to facilitate the downward flow of wood chips through a reactor vessel. U.S. Patent Application Publications 2003/0089470 and 2001/0047854 and U.S. Pat. Nos. 6,199,299 and 5,700,355 disclose examples of vessels having sidewalls that converge in one dimension. The converging sidewalls reduce the cross-sectional area of a vessel and are typically used near the bottom discharge of the vessel. The reduction of the cross-sectional area may not be suitable for upper elevations of a vessel where a generally continuous cross-sectional area is desired to promote uniform flow conditions of the biomass moving down through the vessel.
There is a need for reactor vessels to process biomass, such as annual plant biomass, which facilitate the downward movement of the biomass through the reactor. In particular, the need is for reactor vessels that reduce the tendency of compacted biomass to form channels, stagnant arches and pockets, and to have intermittent or permanent flow stoppages.